In a car-mounted FM receiver apparatus, the received FM reception signal has pulse noise such as ignition noise superimposed thereon, and therefore, for the purpose of canceling such pulse noise contained in the FM reception signal, there is provided a noise canceler.
With a conventional noise canceler, when a composite signal having pulse noise superimposed thereon as shown in FIG. 6A is received, the pulse noise is detected by passing the composite signal through a HPF (high-pass filter). When the HPF detects the pulse noise, a pulse noise detection signal as shown in FIG. 6B is produced. When the pulse noise detection signal is fed to an integrator, the integrator yields an output as shown in FIG. 6C.
Specifically, when the integrator is fed with the pulse noise detection signal, a capacitor included in the integrator is charged, making the output of the integrator higher than a threshold value. When the output of the integrator becomes higher than a predetermined threshold value in this way, the integrator is so controlled that the capacitor is discharged, causing the output of the integrator to decrease gradually. By comparing the output of this integrator and the predetermined threshold value, a gate control signal is produced. With this gate control signal, the operation of a gate circuit for canceling the pulse noise is controlled.
Thus, when the output of the integrator is as shown in FIG. 6C, a pulse noise detection signal is produced, and while the output of the integrator remains higher than the threshold value, the gate control signal remains high. The gate circuit performs signal processing such that the signal level of the composite signal is held at its signal level immediately before the occurrence of the pulse noise. As a result, the composite signals is output after having the pulse noise cancelled therefrom as shown in FIG. 6D.
However, in a noise canceler that cancels pulse noise by operating as shown in FIGS. 6A to 6D, during the occurrence of pulse noise, the gate circuit maintains the signal level immediately before the occurrence of pulse noise irrespective of the state of the reception signal. This causes distortion in the reception signal, resulting in unsatisfactory quality of the sound reproduced therefrom.
Incidentally, Japanese Patent Application Laid-Open No. H8-56168 proposes an FM receiver apparatus that switches filters according to the state of reception, wherein the gate period during which the signal level of the reception signal is maintained to cancel pulse noise is varied so as to achieve appropriate cancellation of the pulse noise. This method, however, is no different from the operations illustrated in FIGS. 6A to 6D in that the pulse noise is cancelled by maintaining the signal level immediately before the occurrence thereof, causing distortion in the reception signal.